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    SubjectRe: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)
    On Wed, Dec 22, 2021 at 02:09:41PM +0100, David Hildenbrand wrote:
    > Right, from an API perspective we really want people to use FOLL_PIN.
    >
    > To optimize this case in particular it would help if we would have the
    > FOLL flags on the unpin path. Then we could just decide internally
    > "well, short-term R/O FOLL_PIN can be really lightweight, we can treat
    > this like a FOLL_GET instead". And we would need that as well if we were
    > to keep different counters for R/O vs. R/W pinned.

    FYI, in my current tree, there's a gup_put_folio() which replaces
    put_compound_head:

    static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
    {
    if (flags & FOLL_PIN) {
    node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
    if (hpage_pincount_available(&folio->page))
    hpage_pincount_sub(&folio->page, refs);
    else
    refs *= GUP_PIN_COUNTING_BIAS;
    }

    folio_put_refs(folio, refs);
    }

    That can become non-static if it's needed. I'm still working on that
    series, because I'd like to get it to a point where we return one
    folio pointer instead of N page pointers. Not quite there yet.

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